A motorcycle tire used on an off-road such as a dirt road generally has a tread pattern provided with a plurality of blocks. Such a motorcycle tire is particularly desired to have a cornering characteristic on a dirt road at a higher level because the cornering characteristic as well as traction and breaking characteristics are considerably important among the various required characteristics. Especially, for a sport tire used in a motocross race or the like, a motorcycle turns at such a high rate of speed so that a lateral gripping force of the tire becomes a critically important factor in determining how well the cornering characteristic is.
However, a conventional motorcycle tire retro fitted especially for dirt road conditions has a basic pattern in which a plurality of width direction block rows consist of a plurality of blocks located along the tire's width direction and the block rows are arranged along the tire's circumferential direction with a space therebetween from the viewpoint of getting better traction. Further, no block is generally placed between the width direction block rows to ensure a good mud expulsion characteristic. As a result, the tire has a poor resistance to a side force in a middle camber region which is most often used during the cornering on the dirt road. It is thus difficult to balance an anti-sideslip characteristic and a traction characteristic. As used herein, the middle camber region means a region between 10 and 40 degrees provided that a upstanding position of a motorcycle is 0 degrees and its horizontally banking position is 90 degrees.
That is, in the conventional motorcycle tire, if the block pitch in the width direction block rows 101 is increased to give a higher negative ratio for an improvement of the mud expulsion characteristic as shown in FIG. 7, the side force resistance is decreased, causing a poor anti-sideslip characteristic. Meanwhile, if a block which has the same height as that of the blocks constituting the width direction block row 101 is arranged between a width direction block row 101 and an adjacent width direction block row 101 for the purpose of enhancing the side force resistance and improving the anti-sideslip characteristic, such a problem occurs that deteriorates the mud expulsion characteristic. It is therefore extremely difficult for the conventional tire to satisfy both the traction and cornering characteristics without involving a deterioration of the mud expulsion characteristic.
Moreover, it is important for an off-road motorcycle radial tire, especially for the one used in motocross, to assure a rigid sidewall due to an attribute of its radial configuration. The tire rim will be hit and damaged primarily by a hard road surface if the rigidity of the sidewall is insufficient. Countermeasures such as simply increasing a case rigidity or increasing the inner pressure in use are effective ways to avoid the rim from being damaged. Such countermeasures, however, may cause a reduction in its capability to absorb bumps and gaps on the road surface, so that the block rigidity of the tread surface is necessarily lower than that of a bias tire, which decreases the gripping performance at the same time. Consequently, it has been extremely difficult for an off-road radial tire to improve its cornering characteristic as well as assure the rigid sidewall without reducing the capability to absorb bumps and the like on the road surface.